In order to produce alternating current power, in particular high-frequency power, for industrial processes, such as induction heating or plasma excitation or excitation of gas lasers, alternating current power production devices having amplifier tubes are often still used for high power levels. One reason for this is the robust nature of the amplifier tubes with respect to rapid load changes. However, alternating current power production devices having amplifier tubes have a poor level of efficiency, and amplifier tubes are subject to wear. Consequently, attempts have increasingly been made to replace these alternating current power production devices with ones that function with semiconductor switching elements. These may, for example, be transistors. Power levels of up to approx. 500 W per transistor can be produced with transistors that are currently available. However, levels of several kilowatts up to megawatts are required. In order to produce such power levels, a plurality of transistors must be connected together to form transistor modules that are installed in power convertor units. In addition, a plurality of power convertor units must be connected together to form alternating current power production devices. Consequently, the number of transistors in an alternating current power production device increases by at least two transistors with each kilowatt required. Consequently, the requirements for the reliability of the individual transistors increase exponentially, since each transistor that fails can lead to the shutdown of an entire alternating current power production device.
In industrial processes, alternating current power production devices are often operated in a pulsed manner, sometimes with very different pulse frequencies of from every second up to a few μs. The transistors are often operated with high frequencies of above 3 MHz. A modulation of the output power is also a known method in industrial processes. In this instance, for example, in induction heating processes, the output power is changed when specific temperatures have been reached in the workpiece and are then intended only to be maintained or changed slightly. When processing workpieces with lasers, for example, the power convertors must bridge relatively long downtimes or standby times in which no power is required, such as when changing workpieces to be processed.
Semiconductor switching elements, such as, for example, transistors, IGBTs, MOSFETS, or transistor modules constructed therefrom, that are operated with high levels of current to produce high levels of power, for example, greater than 100 W, often have a tendency for premature failure, particularly when the operation of the semiconductor switching elements is carried out with pulsed power changes. This is explained in particular by two phenomena. One phenomenon is the different thermal expansion coefficients of the materials used in the semiconductor switching elements (such as, for example, substrate, semiconductor layer, bond wires). Even if all the components of the semiconductor switching element were to heat to a temperature in a uniform manner, the differing thermal expansion of the materials leads to internal mechanical tensions, which, over time and with the expansion movement owing to temperature changes, lead to breakage and failure. Another phenomenon is the differing temperature distribution in particular with intensively cooled components. When producing power of more than 100 W using semiconductor switching elements, it is generally indispensable to cool the semiconductor switching elements in a forced manner, that is to say, for example, by means of cooling members with forced air flow or by means of fluid cooling. A temperature gradient is produced in this instance, for example, from the semiconductor layer of the semiconductor switching elements to the cooling plate. This means that an additional load is also added to the loads described above owing to different temperature distribution. In this instance, there is also still no uniform temperature distribution over the area of the semiconductor switching elements, which also leads to mechanical tensions.
Semiconductor switching elements fail when there are temperature fluctuations. During power production operation, the semiconductor switching elements become warm and the phenomena described above occur. However, between two power operating phases, there is cooling, which leads to further mechanical tensions. A constant change between power operation and pausing between two power operations therefore constantly leads to temperature-related mechanical tensions and movements.